This extension is no longer necessary as facebook rolled out a native dark theme for messenger.com in mid 2024. Follow these instructions to enable it.
I left this up for three months to give people time to switch, but I've since unpublished it from the chrome and edge stores.
You can enable the native dark theme via the preferences in the bottom left menu.
A simple dark theme for Facebook Messenger.
Get it from the Chrome Web Store or the Edge Web Store.
